Modern wireless communication systems employ a number of diversity techniques in order to improve quality and reliability of a wireless communication link. Examples of diversity techniques that are widely used include, but are not limited to, frequency diversity, time diversity and spatial diversity. Each of these techniques transmits data on a plurality of independent channels having different characteristics. Since simultaneous loss of data in each of the plurality of independent channels is less probable, the use of diversity techniques improves error performance of a communication system in spite of several adverse channel effects such as, but not limited to, fading, interference and noise. Consequently, diversity techniques have been extensively used to provide reliable communication across channels with unfavorable conditions.
One of the widely used diversity technique is spatial diversity, also known as antenna diversity. In this diversity technique, multiple spatially separated antennas are used for transmission or reception of data. For example, data may be transmitted by a plurality of spatially separated transmit-antennas towards a receive-antenna. Each path of transmission between a transmit antenna and the receive antenna forms an independent channel carrying the same data. Thus, the use of antenna diversity introduces redundant transmissions of the data. As a result, the error performance of the communication system is greatly improved.
The improved error performance offered by antenna diversity has been successfully exploited in multicarrier communication systems such as OFDM systems. OFDM systems use antenna diversity for both transmission and reception. The most common form of antenna diversity used for transmission at a Base. Station (BS) in an OFDM system is called Transmit Cyclic-Delay-Diversity (CDD). Transmit-CDD is a diversity technique which artificially increases frequency selectivity of a channel by transmitting cyclically delayed versions of the data in addition to the transmission of un-delayed data. Each of the cyclically delayed versions of the data appears as a multipath to a receive-antenna. Effectively, the receive antenna perceives the multiple cyclically delayed versions of data as multipath transmissions from a single transmit-antenna. Therefore, the use of transmit-CDD improves communication while ensuring that the use of multiple transmit-antennas is transparent to the receive-antenna.
In addition to transmit-CDD, a BS also uses antenna diversity technique for reception. The most commonly used antenna diversity technique for reception is Maximum Ratio Combiner (MRC) technique. In this technique, signals derived from multiple receive-antennas at the BS are combined in order to maximize the signal-to-noise ratio. In effect, receive-signal from a receive-antenna at the BS is weighted proportionally by the SNR corresponding to that receive antenna. As a result, reception is improved by enhancing signals from antennas having better SNR.
Usually, the same set of antennas is used at a BS for both transmission and reception. However, the use of different antenna diversity techniques at the BS for transmission and reception results in an asymmetrical channel relationship between the BS and a Subscriber Station (SS).
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